1. Technical Field
The present disclosure relates generally to radio frequency (RF) communications power amplifiers, and in particular, voltage mode power combiners for RF linear power amplifiers.
2. Related Art
Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modulates it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals.
The output of the transmitter is connected to a power amplifier, which amplifies the RF signals prior to transmission via the antenna. The receiver is connected to the output of a low noise amplifier, the input of which is connected to the antenna and receives inbound RF signals. A transmit/receive switch selectively interconnects the antenna to the output of the power amplifier during transmission, and to the input of the low noise amplifier during reception. Thus, the power amplifier, the low noise amplifier, and the antenna switch serves as key building blocks in RF transceiver circuitry. These components may be referred to as a front end circuit.
Conventionally, complementary metal oxide semiconductor (CMOS) technology is utilized for the power amplifier and other front end circuitry. Advancements in these processes have made reduced geometry devices possible, but this has also resulted in such amplifiers exhibiting good linearity only at lower power levels.
In further detail, RF power amplifiers of working communication systems are typically operated over a wide dynamic power range. A conventional Class A power amplifier with typical linearity can meet error vector magnitude (EVM) floor requirements at output power below maximum rated linear power levels, but not over the entire power range of the system. Thus, at higher output power levels, without gain expansion, EVM floor requirements cannot be met. Alternatively, class AB/B amplifiers with gain expansion capability can meet high output power requirements, but not the EVM floor requirements at low to mid power levels.
The graph of FIG. 1, in a first plot 1, shows the upper power/EVM limits of a typical RF digital communications system. A second plot 2 shows the EVM floor of a conventional class AB or B power amplifier over an output power range, where the EVM levels remain within acceptable limits at the higher output power levels, but exceed acceptable limits in the middle range of the output power level. A third plot 3 shows the EVM floor of a conventional class A power amplifier over an output power range, where the EVM levels are lower than the acceptable limits until the higher output power levels, and exceeds the acceptable limits before reaching the upper end of the output power range.
One approach combines a number of power amplifiers with a transformer, as described in publication “A linear Multi-Mode CMOS Power Amplifier with Discrete Resizing and Concurrent Power Combining Structure”, Jihwan Kim et al., IEEE Journal of Solid State Circuits, Vol. 46, Issue 5, pages 1034-1048, May 2011. The power amplifiers are understood to be biased for class A mode. As described in this publication, however, only 14.5 dBm output power can be achieved with an EVM of 1.8%.
Therefore, there is a need in the art for RF power amplifiers with high output power while maintaining linearity. An alternative modality for controlling an RF power amplifier is needed.